Troubleshooting the most common power quality problems

1 From the Fluke Digital Library @ NoteTroubleshooting the most common power quality problemsInstead, the power quality technician or engineer asks, Maybe we should look at the types of loads on the system and monitor for harmonics; perhaps we should monitor for unbalance? Knowing and recognizing the most common power quality symptoms and how to troubleshoot them is a first step in solving power quality issues. What tools do you need for the job?As with any Troubleshooting task, you need the right tools. When it comes to power quality Troubleshooting , these tools may not be what you , you need a good set of up-to-date draw-ings. Then, use a power quality analyzer to measure and record the specific parameters associ-ated with power quality . Other tools, such as a data logger, thermal imager, infrared thermometer, and recording digital multimeter, can also aid in Troubleshooting .

2 What kinds of problems will you find? common power quality problems are grouped into two broad areas: voltage anomalies and harmonic distortion issues. Voltage anomalies can cause sev-eral problems , many easily corrected. The key is to spot the dips or sags are responsible for up to 80 percent of all power quality issues. A dip or sag occurs when the system voltage drops to 90 percent or less of nominal system voltage for a half-cycle to one minute. common symptoms of dips include incandescent lights dimming if the dip lasts more than three cycles, computer lockup, spu-rious shutdown of sensitive electronic equipment, data (memory) loss on programmable controls, and relay control problems . To troubleshoot potential dip problems , begin by monitoring at the load where the dip symptoms first occur.

3 Compare the time of the equipment s operational failure to the time at which the volt-age dip occurred; if there is not a correlation, the problem is most likely not voltage dip. Continue Troubleshooting by monitoring farther upstream until the source is located. Use plant one-line drawings to help determine whether starting large motors is creating the dip, or whether there are other sudden increases in current requirements in the plant. Tools and tips for issues with voltage distortion and harmonicsTroubleshooting is a systematic process of finding and eliminating problems . To the untrained eye, problems in electrical distribution systems may not be recognizable as power quality problems . For example, a tripped thermal-magnetic circuit breaker typically indicates a short circuit, ground fault, or overload.

4 When no immediate problem is apparent, it may be written off as just an old breaker that needs replacing. 2 Fluke Corporation Troubleshooting the most common power quality problemsVoltage swells or surges occur only about half as often as dips. However, increases in system voltage for short periods up to a cycle or more can cause problems . As with all power quality prob - lems , you must monitor parameters for a period of time, then observe and interpret. Symptoms of swells often include immediate failure of equipment, typically the power supply section of electronics. However, some equipment failures may not occur immediately, because volt-age swells can occur over a period of time and prematurely break down components. If analysis of electronic equipment reveals faulty power supplies, monitor voltage trends on the feeders and branch circuits feeding this equipment.

5 Where possible, compare failure rates of similar equipment oper-ating on portions of systems known not to be experiencing analyzing power quality survey results, look for any sudden line-to-ground faults on a single-phase line. This type of fault causes the voltage to suddenly swell on the two non-faulted phases. Large plant loads suddenly dropping offline, and power factor correction capacitor switching, can also cause voltage transients can cause symptoms rang-ing from computer lockups and damaged electronic equipment to flashover and damaged insulation on distribution , sometimes referred to as spikes, are substantial increases in voltage but only for a matter of microseconds. Lightning strikes and mechanical switching are common causes. Equipment failure during a storm is often right-fully attributed to transients and no power quality monitoring is causes of transients include switching of capacitors or capacitor banks, reenergizing systems after a power failure, switching of motor loads, turning off or on fluorescent and HID lighting loads, switching transformers, and sudden stoppage of certain equipment.

6 For these transient conditions, monitor at the load and correlate equipment opera-tional problems or failure with distribution system arcing across contacts by interrupting large loads can be a cause of transients. Use the facility one-line to move the monitoring farther upstream in the distribution system until you find the interruptions can last anywhere from two to five seconds or more. The symptom is usu-ally quite simple: the equipment stops operating. Interruptions for longer than five seconds are typi-cally referred to as sustained interruptions. most motor control circuits and process control systems are not designed to restart after even a brief inter-ruption of a voltage interruption occurs when equip-ment is unattended, the cause of the equipment shutdown might not be properly identified.

7 Only monitoring the equipment and correlating the time of any power interruptions to the time of equip-ment issues will help identify voltage interruptions. Voltage unbalance is one of the most common problems on three phase systems, and can result in severe equipment damage, yet it is often overlooked. For example, a voltage unbalance of percent on a 230 V motor results in a current unbalance of almost 18 percent, causing a tem-perature rise of 30 C. While a digital multimeter (DMM) and some quick calculations can be used for averaging voltage readings, a power quality analyzer provides the most accurate information about can occur at any point throughout the distribution system. Loads should be equally divided across each phase of a panelboard. Should one phase become too heavily loaded in compari-son to others, voltage will be lower on that phase.

8 Transformers and three-phase motors fed from that panel may run hotter, be unusually noisy, vibrate excessively, and even suffer premature a power quality analyzer, such as this Fluke 435 Series II power quality and Energy Analyzer, to measure and record the specific parameters associated with power Fluke Corporation Troubleshooting the most common power quality problemsFluke Corporation PO Box 9090, Everett, WA 98206 Europe PO Box 1186, 5602 BD Eindhoven, The NetherlandsFor more information call: In the (800) 443-5853 or Fax (425) 446-5116 In Europe/M-East/Africa +31 (0) 40 2675 200 or Fax +31 (0) 40 2675 222 In Canada (800)-36-FLUKE or Fax (905) 890-6866 From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116 Web access: 2013 Fluke Corporation.

9 Specifications subject to change without notice. Printed in 8/2013 6000499A_ENModification of this document is not permitted without written permission from Fluke The most Trusted Tools in the reality, voltage differences between phases vary as loads operate. However, motor or trans-former overheating, or excessive noise or vibration, can merit Troubleshooting for voltage over time is the key to capturing unbalance. In a three-phase system, the maximum variation in voltage between phases should be no more than 2 percent (the Vneg % value on the analyzer), or significant equipment damage can are voltages and currents whose frequency is said to be an integer multiple of the fundamental frequency. For example, the third har-monic is the voltage or current that is occurring at 180 Hertz (Hz) in a 60 Hz system (3 x 60 Hz = 180 Hz).

10 These unwanted frequencies cause numer-ous symptoms, including overheating in neutral conductors and the transformers supplying these circuits. Reverse torque creates heat and efficiency losses in most severe symptoms created by har-monics are typically the result of the harmonics distorting the fundamental 60 Hz sine wave found in facilities. This sine wave distortion results in improper operation of electronic equipment, spurious alarms, data losses, and what are often reported as mysterious symptoms of harmonics occur, trouble-shoot by observing total harmonic distortion (THD). Significant increase in THD under varying load conditions warrants a percentage comparison of each individual harmonic current level as com-pared to the total fundamental current flow in the system.